Spread spectrum communication systems, such as frequency hopping (FH) communication systems, have excellent properties, such as high resistance to interference and data security. Such communication systems are known to be used in communication fields, such as satellite communication and ground communication, and are progressively being applied in mobile communication and local communication.
FH communication systems employ a wide bandwidth to perform communication. Such communication systems switch or modulate a carrier wave for data signals into different frequencies in accordance with spreading codes. An example of a conventional FH communication apparatus is briefly described below with reference with FIG. 12.
In a transmission operation, transmit data are composed of analog signals that are encoded in a coding circuit 1. The encoded data are modulated in a modulating circuit 2 through a modulating method, such as frequency shift keying (FSK) or a phase shift keying (PSK). The modulated data are mixed with outputs of a frequency synthesizer 8 in a mixer 3. Synthesizer 8 changes its output frequency in accordance with spreading codes (e.g., a frequency hopping pattern), which are generated by a spreading code generating circuit 7. The mixed data are amplified in a transmit circuit 4 and sent from an aerial 6, via a duplexer 5 that alternates between outputting the transmit data and inputting receive data from another FH communication device.
In a receive operation, signals received via aerial 6 are input into a receive circuit 11 through duplexer 5, and are amplified through a bandpass filter of receive circuit 11. The amplified signals are mixed with outputs of synthesizer 8 in a mixer 10. The frequency output from synthesizer 8 is hopped in accordance with a spreading code (e.g., a frequency hopping pattern), generated by spreading code generating circuit 7. A synchronous circuit 9 is adapted to obtain and hold synchronization for the switching of the frequency, and synchronizes the received signals with the output frequency of synthesizer 8. The synchronized outputs of mixer 10 are demodulated into binary data in a demodulating circuit 13, and are decoded into analog receive data in a decoding circuit 12.
A synchronizing operation of synchronous circuit 9 is generally described below. The receiving side apparatus initiates a preliminary synchronization by selecting a frequency ("selected frequency") from the hopping frequencies of a prescribed hopping pattern. The receiving side apparatus then determines whether the frequencies following the selected frequency match the received frequencies. If so, the receiving side apparatus continues modulating provisionally according to the hopping pattern (also referred herein as "hopping sequence"). If the received hopping frequencies continue to match the detected frequencies over one hopping cycle, after the preliminary synchronization, then FH communications have been synchronized between the transmitting-side apparatus and the receiving-side apparatus. Thereafter, the receiving-side apparatus activates a synchronization holding process to maintain the synchronization.
As described above, conventional FH communication systems vary carrier frequencies for spreading, meaning that a narrow-band transmission is seen at each period of an FH cycle. Accordingly, when a carrier frequency coincides with an existing communication frequency, some narrow-band jamming frequency or a hopping frequency of other FH communication devices, the signals transmitted on the carrier frequency may be degraded, thereby resulting in error signals at the receiving side apparatus. Interference frequencies may also coexist in a hopping frequency, resulting in degradation of the transmit signals. Furthermore, conventional FH systems may not be able to receive signals of a certain frequency band due to multi-path phasing.
Accordingly, there is a need to provide error detection and correction techniques in FH communication systems to remedy the above problems. In particular, there is a need to modify a hopping pattern in the event of reception errors.
One approach to remedying the above problem is found in U.S. Pat. No. 5,541,954 to Emi, which discloses a frequency hopping communication method and apparatus for changing a hopping frequency as a result of a counted number of errors. When a coding circuit informs a spreading code control unit of an error generation, it specifies an error prone carrier frequency and counts the errors for each hopping frequency. If the counted errors exceed a fixed value for a particular frequency, the spreading code control unit changes the particular frequency to another unused frequency and informs a data communication control unit of the change. The data communication control unit notifies the other party apparatus of the frequency change, via control signal. The other party apparatus receives the control data, changes the particular frequency to a corresponding new one, and resumes data transmission with the modified hopping pattern.
However, one problem with the Emi system is that errors affecting a particular transmission frequency may also affect neighboring frequencies, particularly, those that are being employed in the hopping pattern or are selected to replace the error-affected hopping frequency (hereinafter "erred frequency"). In such a case, the Emi system needs to replace the neighboring frequencies, thereby requiring communication to be stopped until a new sequence of frequencies can be transmitted to the other party. As a result, in such instances, the overall throughput rate of the communication system may be significantly degraded.
Another problem with the Emi system and other conventional FH communication systems is that such systems transmit FH modulated signals for all hopping frequencies at the same transmission power level. Furthermore, they operate at unnecessarily high transmission power levels to ensure that the transmitted signals will reach the receiving side device at a reasonable level. As such, these systems operate at inefficient power consumption levels. Such systems further transmit signals at transmission power levels that may interfere with other neighboring systems and pose a data security risk (e.g., the data is transmitted beyond a necessary transmission range, thus increasing the possibility of unauthorized access to the data).
There is a need to provide an apparatus and method that performs FH communication over less error prone hopping frequencies. There is also a need to provide an FH communication apparatus and method that minimizes the number of frequency hopping replacement operations and, more specifically, provides rapid and efficient elimination of erred areas or portions of an available frequency spectrum. Furthermore, there is a need to provide an FH communication apparatus and method that is capable of controlling the transmission power levels to optimize power consumption, minimize communication interference with neighboring devices and increase data security.
Accordingly, it is an object of the present invention to provide an efficient apparatus and method for error detection and link quality improvement in an FH communication system.
It is a further object of the present invention to provide an FH apparatus and method that employs efficient error detection techniques, minimizes the amount of FH replacement operations and provides rapid and efficient elimination of erred areas or portions of an available frequency spectrum.
Another object of the present invention is to provide an apparatus and method thereof for minimizing reoccurrence of transmission errors according to previously detected errors.
A further object of the present invention is to provide an FH communication apparatus and method that is capable of controlling the transmission power levels to optimize power consumption, minimize interference with neighboring devices and increase security.
Another object of the present invention is to provide an FH communication apparatus and method that optimizes transmission power level for each carrier frequency in a hopping pattern.
It is also an object of the present invention to provide an FH communication apparatus and method that is capable of automatically controlling the transmission power levels of each hopping frequency.